#lang racket
(define (cbrt x)
   (define delta 0.0001)
   (define (cbrt-iter guess x)
     (if (good guess x)
         guess
         (cbrt-iter (improve guess x) x)))
   (define (good guess x)
     (< (/ (abs (- (improve guess x)
                   guess))
           guess)
        delta))
   (define (improve guess x)
     (/ (+ (/ x (* guess guess))(* 2 guess)) 3))
  (cbrt-iter 1.0 x))

;test
(cbrt 27)